Mass spectrometry



March 28, 1961 c. F. ROBINSON MASS SPECTROMETRY 2 Sheets-Sheet 1 FiledApril 14, 1955 INVENTOR. CHARLES f. ROBINSON AJTORNEVS m, Wuhan/4 M 2Sheets-Sheet 2 Filed April 14, 1955 N Kw M mw & we VP 1% E H w m W m Y Bm 6Q w 5&3 cw Ekom uq w nll 1W h a 9 E30 E T whim 2 0 E30 E N mx x ATTORNEVS United States Patent assignments, to ConsolidatedElectrodynamics Corporation, Pasadena, Calif., a corporation ofCalifornia Filed Apr. 14, 1955, Ser. No. 501,289

4 Claims. (Cl. 250-413) This invention relates to mass spectrometry andparticularly to improvement in the methods of operation of a cycloidalor crossed field mass spectrometer.

The principle of operation of a cycloidal mass spectrometer is asfollows: If a charged particle is introduced into a magnetic field itwill move in a circular path to return to its point of origin. This istrue regardless of the mass of the particle, with particles ofincreasing mass traveling in circles of increasing radius, but in eachinstance returning to the point of origin. If auniform electric field isimposed across the space defined by the magnetic field and normal to themagnetic field, the ions pursue a path which may beconsidered asrigorously circular in a coordinate system moving with uniform velocity.The movement of the coordinate system is a.

function of the ratio of the electric and magnetic field strengths. Ifions of a particular mass are introduced into such a field system theywill complete one turn of their circular motion in a time which dependsdirectly on the mass of the particle and, if the electric field strengthis uniform so that the coordinate systems corresponding to each particlemove at the same velocity, the particles will converge to a series ofrigorous point foci after any integral number of turns in the magneticfield and regardless of their velocity or direction of travel at themoment of introduction into the field. Since the.

acteristic advantage. ,Because of the-fact that ions of a given massWill converge to a rigorous point foci after any integral number ofturns regardless of their velocity or direction of travel at the momentof introduction into the crossed field, the instrument is not subject toaber-, ration and is insensitive not only to the energy spread of ionsintroduced into the field but also to the angular divergence of the ionbeam at the point of introduction into the field.

The present invention takes advantage ofthis unique characteristic of acycloidal mass spectrometer'in an unusual manner of operating the ionsource. The invention involves the distribution of potentials in the ionsource such that the ions are propelled from the point of formation intothe'region of crossed fields under diiferent influences than areconventionally possible in;

an ion source. The region of crossed fieldsin the cycloidal'rnassspectrometer is referred to herein as the cycloidal analyzer.

In terms to structure and {electrode arrangement, the

ion source with which the inve'ntion is involved may.

include the usual repeller electrode, a first apertured 2,971,476Patented Mar. 28, l9l51 accelerating electrode, means for developing anionizing; electron beam traversing the region between the repeller andfirst accelerating electrode, and a second apertured; acceleratingelectrode, the apertureof which represents an exit slit through whichions emerge from the ion systern into the cycloidal analyzer, and meansfor supplying potentials to these electrodes. 7 The present inventioncontemplates a method of mass. spectrometry comprising the formation ofions in an ionizing region by means of an electron beam, theboundariesof which define the ionizing region, propelling theions; into ananalyzer region spaced from the ionizing region; by means of anelectrical propelling field of such configuration that the fieldstrength in the ionizing region is at least 25% as high as the fieldstrength at any point, between the ionizing region and the analyzerregion maintaining in the analyzer region transversely oriented;magnetic and electrical fields whereby ions entering the region arecaused to pursue cycloidal trajectories with ions of any given massconverging at a point after one; cycle of motion and collecting theconvergent ions of a given mass. I Q j In essence the inventioninvolvesmaintaining a ,field' strength in the region ionization many,many times, greater than that possible informs of mass spectrometryvother than the double focusing cycloidal instruments; Another way ofexpressing this relationship is to state, that the repeller voltageestablished between the repeller electrode and the first acceleratingelectrode, this being; the potential developing arepelling'field in thisregion, is at least one-fourth as great as the injection voltage, this,being defined as the potential existing between the median;

plane of the ionizing electron beam and the point of: 7

entry of the ions into the cycloidal analyzer, i;e. the aper ture in thesecond accelerating electrode. In mass spectrometers other than thecycloidal mass spectrometer with whichthis invention is concerned effortis made to main-; tain the electric field in the region of ionization assmall as, practical. This is because these other types of mass.spectrometers are extremely sensitive to dilierences in, the initialenergies of the ions. Since the ionizing electron beam has a finitewidth, the greater the potential duces these elfects by minimizing'theresidence time pf: 70.

the electron beam- This high field strength, whichemay be tentimes thatacceptable'in other forms ofmass I spectrometers, greatly reducesadverse space charge, effects, the'magnitude of which is proportionalto'the;

residence time of ions in the ion source and hence in versely, relatedto the magnitude of the repellingfie ldq, Also,'subjecting1the ionsto'potentials as herein'defined,

reduces discrimination effects. In'any mass spectrorneter ion source,conditions are adjusted so that a particu lar ion mass will emergetherefrom under conditions which are'optimnm for that ion. a The sameset of cond tions will not be optimum for another ion which difiel'sappreciably in mass or in initial velocity from the first; so that it isinherent that any source, will show discrimi y nation between ions ofdifierentmass.

It follows that the longer the ions remain in the ome; the greater isthe opportunity for the ionsource to create these discriminationeffects. ,The present invention}:

the ions in the source. v fl'he invention will be clearly understoodfrom thefollowing detailed description taken inconjunction with theaccompanying drawing, in which;

Fig. 1 is a longitudinal section elevation through a cycloidal focusingmass spectrometer;

Fig. 2 is a schematic diagram of the electrical circuitry of thespectrometer of Fig. 1; and

1 Fig. 3 is a greatly enlarged schematic view of an ion source showingthe critical conditions above defined.

The cycloidal mass spectrometer shown in Fig. 1 comprises an evacuableenvelope 5 provided with conduit means 6 for connection to an evacuatingsystem (not shown) and sample inlet means 7. A plurality of electrodes8, 9, 10, 11, 12, 13 and 14 are supported in the envelope from aframework 15 by a series of pins 16, 17, 17A, 18, 19, 20. The severalelectrodes are spaced and insulated from each other by insulatingspheres 21, 22, 23, etc. The electrode structure defines a chamber 24having an inlet slit 25 and a resolving slit 26 spaced from each otheron a common or so-called focal plane. An ion source 28 is supportedadjacent the chamber 24 and includes a terminal accelerating electrode29 definin the above mentioned inlet slit 25.

As shown schematically in Fig. 2, the ion source includes a chamber 30,electron gun 31, an electron target 32, a repeller electrode 33, and afirst accelerating electrode 34 having an aperture '35 and the electrode29 defim'ng the inlet aperture 25 to the cycloidal analyzer. The aboverecited elements of the ion source are arranged so that molecules in thechamber 30 are ionized by electron beam 36 traveling between the gun 31and target 32 and, under the influence of the potential between therepeller electrode 33 and the accelerating electrodes 34 and 29, theions are propelled through aperture 25 into the cycloidal analyzer.

Electron gun 3'1 and target 32 are conventionally interconnected throughan emission regulator circuit 38 so that the ionizing electron beam 36is maintained at substantially uniform density. Many emission regulatorcircuits are known in the art as conventional adjuncts to a great numberof commercial mass spectrometers.

Appropriate potentials are impressed on the several electrodes 8, 9, 10,etc. by means of a voltage divider network also shown schematically inFig. 2. A D.C. power supply 40 is connected across a capacitor 41. Avoltage divider 42 is connected in parallel across the capacitor 41 andthe several electrodes 8, 9, 10, 11, 12, 13 and 14 are connected to thedivider network 42 as illustrated. A mass spectrum can be scanned bycharging the capacitor 41 and allowing the charge to decay across thevoltage divider 42. The several field forming electrodes will remain atthe same relative potentials but the field strengths will diminish asthe capacitor 41 discharges and different ion trajectories will bebrought to focus at the resolving aperture 26.

Electrode 12 is provided with a cavity 44 into which the resolvingaperture 26 opens and in which a collector electrode 45 is mounted. Ionsfocusing on the resolving aperture 26 will collect on and discharge atthe collector electrode 45. Suitable electrical leads are broughtthrough a wall of the envelope 25 in conventional manner for connectionto the various portions of the ion source of the field formingelectrodes and the collector.

electrode. An electrical conduit 47 is shown as accomplishing thispurpose, the individual electrical leads brought into the conduit notbeing distributed in the drawing for purposes of clarity.

The entire instrument is immersed in a magnetic field developed betweenmagnet pole 48 and a companion pole piece (not shown) disposed on theopposite side of the envelope 5. The two magnet poles develop a magneticfield normal to the electrical field existing between the severalelectrodes 8, 9, 10, etc.

Operation of the mass spectrometer in accordance with the presentinvention is best described with relation to the enlarged schematic viewof a typical ion source as shown in Fig. 3. The source includes repellerelectrode 33, electron beam 36 shown in cross-section, first acceLlustrated and described in detail.

erating electrode 34 provided with aperture 35 and the secondaccelerating electrode 29 provided with the aperture 25, this being theinlet slit into the cycloidal analyzer. Various notations areillustrated on the drawing relative to potentials established in the ionsource. The potential existing between the median plane of the electronbeam 36 and the inlet aperture 25 is known as the injection voltage,represented by the notation V this being the potential influence towhich the average ions formed in the electron beam are subjected. Therepeller voltage is that potential existing between repeller electrode33 and accelerating electrode 34 and is designated by the symbol, V Acomponent of the injection voltage is that potential existing betweenelectrodes 34 and 29 and is designated by the symbol V In accordancewith the invention, a mass spectrom ter as illustrated in Fig. 1, thatis, a cycloidal type mass spectrometer having an ion source of the typeshown schematically in Fig. 3, is so operated that the repeller voltageV is at least equal to one-fourth the injection voltage V; andpreferably to one-half the injection voltage V;. This means that thefield strength in the ionizing region, that is, in the region offormation of the ions defined by the boundaries of the electron beam, isat least one-fourth as high as the field strength anywhere else in theion source, and again preferably one-half as high as the field strengthanywhere else in the ion source. As above explained, the result of theseconditions is that the ions at the moment of formation are subjected toa repelling potential approximately ten times the magnitude of thatacceptable in other forms of mass spectrometry, and accordingly areremoved from the source a great deal more rapidly. This also means thations formed adjacent the rear boundary of the electron beam will havemuch greater initial energies than those formed adjacent the forwardboundary of the electron beam.

Typical values of operating potentials in accordance with the inventionare as follows:

Magnetic field strength=3400 gauss VI:

For m/e :8.4 volts For m/e 16:52.5 volts VRI For m/e 100:4.2 volts Form/e 16:26.25 volts In the operation of the illustrated device, samplemole, cules introduced to the ion source are ionized by the electronbeam and, under theinfiuence of the injector, are expelled from thesource into the cycloidal analyzer. Responsive to the transverselyoriented magnetic and electrical fields impressed across the analyzer,the ions pursue cycloidal trajectories in the chamber. The pitch of theion trajectories is a function both of mass and electrical and magneticfield strengths, the latter being controlled to focus ions of a givenpredetermined mass at the resolving slit. The in-focus ions arecollected at the collector electrode and the resultant discharge currentis sensed in any conventional fashion. To scan a mass spectrum, eitherof the transverse magnetic and electrical fields may be varied tosuccessively focus ions of a different mass on the resolving slit. Theion source meeting the structural limitations herein specified greatlyimproves the operation of an instrument of this type in avoiding iondiscrimination normally encountered in the source, reducing thesensitivity of the instrument to space charges in the ion source andincreasing the absolute sensitivity of the instrument by passing agreater number of the total formed ions in the source into the cycloidalanalyzer.

One form of cycloidal mass spectrometer has been il- However, it isunderstood that the invention isnot directed to the specific form ofinstrument as presented apart from the specific construction of the ionsource as recited. The particular ion source is applicable to any of themany forms of cycloidal mass spectrometers, the source being limitedonly to use with a double focusing instrument of this particular generictype.

I claim: a

1. In a cycloidal mass spectrometer having an ion source, an analyzerseparated from and communicating with the source through a firstapertured accelerating electrode, means for forming in the analyzercrossed magnetic and electric fields, a repeller electrode in the ionsource spaced from said first apertured accelerating electrode, a secondaccelerating electrode disposed in the source between the repellerelectrode and the first accelerating electrode and means for directingan ionizing electron beam through the ion source between the repellerelectrode and second accelerating electrode, the combination comprisingmeans for developing a relatively high repeller voltage V between therepeller electrode and the second accelerating electrode, means fordeveloping a voltage V between the second accelerating electrode and thefirst accelerating electrode, means for establishing the voltages V andV such that the voltage V is at least A the magnitude of the injectionvoltage V which is that voltage existing between the median plane of theelectron beam and the first accelerating electrode and the voltages Vand V comprise the components of the injection voltage, and means forestablishing a strong magnetic field in the ion source to collimate theelectron beam to compensate for the relatively high repeller voltage.

2. In a cycloidal mass spectrometer having an ion source, an analyzerseparated from and communicating with the source through a firstapertured accelerating electrode, means for forming in the analyzercrossed magnetic and electric fields, a repeller electrode 'in the ionsource spaced from said first accelerating electrode, a second aperturedaccelerating electrode disposed in the source between the repellerelectrode and the first accelerating electrode and means for directingan ionizing electron beam through the ion source between the repellerelectrode and second accelerating electrode, the combination comprisingmeans for developing a relatively high repeller voltage V between therepeller electrode and the second accelerating electrode, means fordeveloping a 'voltage V between the second accelerating electrode andthe first accelerating electrode, means for establishing the voltages Vand V such that the voltage V is at least /2 the magnitude of theinjection voltage V which is that voltage existing between the medianplane of the electron beam and the first accelerating electrode and thevoltages V and V comprise the components of the injection voltage, andmeans for establishing a magnetic field in the ion source to collimatethe electron beam to compensate for the relatively high repellervoltage.

3. In a cycloidal mass spectrometer having an ion source, an analyzerseparated from and communicating with the source through a firstapertured accelerating electrode, means for forming in the analyzercrossed magnetic and electric fields, a repeller electrode in the ionsource spaced from said first apertured accelerating electrode, a secondaccelerating electrode disposed in the source between the repellerelectrode and the first accelerating electrode and means for directingan ionizing electron beam through the ion source between the repellerelectrode and second accelerating electrode, the improvement whichcomprises the steps of establishing a relatively high repeller voltage Vbetween the repeller electrode and the second accelerating electrode,establishing a voltage V between the first and second acceleratingelectrodes, maintaining the voltages V and V such that the voltage V isat least one-quarter the magnitude of the injection voltage V which isthe 'voltage existing between the median plane of the electron beam andthe first accelerating electrode, the components of the injectionvoltage V; comprising V and V and simultaneously producing a strongmagnetic field in the ion source to collimate the electron beam tocompensate for the co-existing relatively high repeller voltage.

4. In a cycloidal mass spectrometer having an ion source, an analyzerseparated from andcommunicating with the source through a firstapertured accelerating electrode, means for forming in the analyzercrossed magnetic and electric fields, a repeller electrode in the ionsource spaced from said first apertured accelerating electrode, a secondaccelerating electrode disposed in the source between the repellerelectrode and the first accelerating electrode and means for directingan ionizing electron beam through the ion source between the repellerelectrode and second accelerating electrode, the improvement whichcomprises the steps of establishing a relatively high repeller voltage Vbetween the repeller electrode and the second acceleratingelectrode,establishing a voltage V between the first and second acceleratingelectrodes, maintaining the voltages V and V such that the voltage V isat least one-half the magnitude of the injection voltage V; which is thevoltage existing between the median plane of the electron beam and thefirst accelerating electrode, the components of the injection voltage Vcomprising V and V and simultaneously producing a strong magnetic fieldin the ion source to collimate the electron beam to compensate for theco-existing relatively high repeller voltage.

References Cited in the file of this patent UNITED STATES PATENTS

